Other Talks in the Series: Protein Epidemiology

Hello, my name is Paul Kim, and I'm an assistant professor at McMaster University in Canada. This seminar will focus on the "Advances in Fibrinolysis", mostly discussing the basic science aspects of fibrinolysis research. I will also discuss clinical application of these findings and thrombolytic therapy.
Coagulation and fibrinolysis exist as a fine hemostatic balance. The left side represents the coagulation cascade, which is shown by the conversion of prothrombin to thrombin. Thrombin, the central enzyme in coagulation cleaves soluble fibrinogen into insoluble fibrin clot. The right-hand side represents the fibrinolytic cascade, which is shown by the conversion of plasminogen to plasmin. Plasmin, the essential enzyme in fibrinolysis digests insoluble fiber in clots to soluble fibrin degradation products, or FDPs.
The loss of this balance leads to various pathological outcomes; uncontrolled up regulation of the coagulation cascade leads to thrombotic diseases such as deep vein thrombosis, heart attacks, and ischemic strokes.
Alternatively, uncontrolled up regulation of the fibrinolytic cascade leads to bleeding episodes or hemorrhaging.
Therefore, the positive feedback systems inherent to both the coagulation and fibrinolytic systems are regulated by inhibitory mechanisms. Thrombin generation is down-regulated by the activation of protein C to form activated protein C. Plasmin generation is down-regulated by the activation of thrombin activatable fibrinolysis inhibitor, or TAFI, to generate TAFI a. Central to activation of protein C and TAFI is the thrombin thrombomodulin complex. Thrombomodulin, a trans-membrane protein expressed on the surface of endothelial cells, bind with thrombin, which then act to alter the substrate specificity of thrombin from a pro-coagulant state to an anti-coagulant state.
So, let's begin with fibrinogen, the soluble precursor to fibrin clots. It consists as a dimer of three chains: alpha, beta, and gamma, that are all cross-linked to form a single dumbbell-shaped molecule. This diagram indicates the various domains of each fibrinogen molecule, with the shaded area representing the D domain. And the middle region, where the end terminus of the six chains come together, representing the E domain.
Once fibrinogen is cleaved by thrombin and thus removing the fibrinal peptides a and b from the alpha and beta chains respectively, fibrin monomers are generated. These monomers then line up in a half staggered bilayer, much like laying bricks. Whereby the two D domains of adjacent fibrin set opposite from the E domain of the parallel strand. In addition, Factor XIII is activated by thrombin to form the transglutaminase Factor XIIIa, which can then cross-link the adjacent D domains, thus stabilizing the fibrin clot. To remove these clots, the protofibril strands are cleaved by a plasmin. Because each protofibril strand consists of at least two parallel strands, complete cleavage of any protofibril strand requires a minimum of six cleavages by plasmin at any given location. Depending on the cleavage location, varying fragments can be generated as shown in the diagram, with the smallest fragment being the DDE unit commonly referred to as the D-Dimer.
So how is fibrin breakdown initiated? It is triggered by the activation of plasminogen to plasmin by the enzyme tissue type plasminogen activator, or t-PA. The generator plasmin then digests fibrin to a partially digested fibrin also referred to as plasmin modified fibrin, designated FM prime. Interestingly, fibrin acts as a co-factor in enhancing its own breakdown by enhancing EPA mediated plasminogen activation by three orders of magnitude, compared with t-PA alone. This is due to fibrin acting as a template onto which plasminogen and t-PA can form a ternary complex. Plasmin modifying fibrin further enhances plasminogen activation by t-PA that results in an even faster clot breakdown. This is achieved when new C-terminal lysine and arginine residues are made upon proteolysis of fibrin strands by plasmin, which acts as new binding sites for plasminogen and t-PA. As mentioned before, this process is regulated by TAFIa, which is activated from its precursor, TAFI, by the thrombin thrombomodulin complex. TAFI is also referred in literature as procarboxypeptidase URB2 as well as plasma procarboxypeptidase B. Once activated, TAFIa then further modifies FN prime to TAFIa modified fibrin designated FN double prime. This reduces the cofactor activity of FN prime by nearly 100-fold. This is achieved by TAFIa and carboxypeptidase removing the newly exposed carboxyl lysine and arginine residues. Thereby reducing plasminogen and t-PA binding sites that were generated by a plasmin.

Advances in fibrinolysis

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